1. Technical Field
The present invention relates to a method of locating the position of a work in an automatic exposing apparatus. More particularly, the present invention is directed to a method of locating the position of a work in an automatic exposing apparatus, which is preferably used for automatically aligning a copper-plated laminar plate to be exposed with a mask film when wiring patterns on a printed wiring board are to be exposed.
2. Description of the Related Art
An automatic exposing apparatus of the type including an automatic aligning unit adapted to perform image recognizing and figure processing is generally classified into one of two types. One type being such that wiring patterns are automatically aligned with a work to be exposed to light beam such as a copper-plated laminar plate in an exposing chamber. The other type being such that a work is previously automatically aligned at a location other than an exposing chamber and thereafter it is introduced into the exposing chamber, as disclosed in the official gazette describing Japanese Laid-Open Patent NO. 46466/1988. The second method is practiced in such a manner that the work is previously formed with alignment marks, the work being fixed to an alignment table at the time of automatic aligning. The alignment marks are optically read by cameras (serving as image receivers) and the alignment table is then displaced such that the alignment marks are located in correspondence to predetermined positions when the work is introduced into the interior of the exposing chamber, whereby the work position is located, i.e., automatic aligning is effected. It has been found that this method is preferably used for carrying out mass production at a high efficiency.
The second method is further classified into two types. The first type being such that, e.g., in a case where wiring patterns on the lower surface of a copper-plated laminar plate serving as a work are exposed to a light beam, cameras for optically reading alignment marks on the copper-plated laminar plate are arranged above the copper-plated laminar plate. The second type being such that the cameras are arranged below it. Arrangement of the cameras above the copper-plated laminar plate provides an advantage in that alignment marks can freely be placed at positions which are convenient for allowing them to be optically read by the cameras irrespective of the position where the wiring patterns to be exposed are located, because the alignment marks are placed on the opposite surface to a surface to be exposed. However, it has a problem in that an error is liable to occur in alignment accuracy, because the alignment marks placed on the opposite surface to the exposure one are utilized for the purpose of locating. Especially, in a case where exposing is repeatedly effected for several times with the same work, alignment accuracy is reduced substantially. In contrast with the foregoing case, arrangement of cameras below the exposure surface of a copper-plated laminar plate leads to other problems. For example, the positions where alignment marks are placed on a work and the positions where the cameras are installed are restricted, because of the necessity for optically reading the alignment marks placed at a position where an error hardly occurs due to warp of the copper-plated laminar plate fitted to an alignment table. A degree of allowance relative to positions of the alignment marks placed on the work and positional changing of the cameras is remarkably reduced, when the shape of a copper-plated laminar plate is changed or wiring patterns to be exposed are changed and consequently, versatility inclusive adaptability to all operative conditions is lost.